The invention is concerned with ion implanters and in particular a method of obtaining a performance parameter for an ion implanter and ion implanters arranged for performing this method.
Ion implanters have a vacuum enclosure in which a beam of ions for implantation is generated. The beam is directed at a wafer to be implanted which is mounted on a holder in a process chamber of the vacuum enclosure. A scanning arrangement effects repeated relative scanning of the ion beam over the wafer to complete an implant cycle. In batch implanters, a number of wafers (a batch) is processed simultaneously in a single implant cycle. Typically, the batch of wafers is mounted around the periphery of a wheel like structure in the process chamber and the wheel is rotated so that the wafers are sequentially scanned across the ion beam. At the same time, the axis of rotation of the wheel is reciprocated to and fro so that ions are implanted evenly over the exposed surfaces of the wafers.
Single wafer serial implanters are also known in which individual wafers are treated with a complete implant cycle one after the other. A typical batch type implanter is described in U.S. Pat. No. 5,389,793, and a single wafer serial implanter is described in U.S. Pat. No. 5,898,179.
The arrangement for relative scanning of the ion beam and the wafer mounted on the wafer holder may be fully mechanical, as in the batch type implanter disclosed in the above referred U.S. Pat. No. 5,389,793, or may be a hybrid of mechanical scanning in one direction and scanning of the ion beam in the other direction, as described in U.S. Pat. No. 5,898,179. Furthermore, two dimensional beam scanning systems are also known in which the wafer is held substantially stationary whilst the beam itself is scanned in two orthogonal directions to complete the implant cycle. The present invention is relevant to implanters with any beam scanning arrangement, whether mechanical, hybrid or beam scanning (whether electrostatic or electromagnetic), as applied to both batch and single wafer serial type implanters.
For successful operation of the ion implanter, the vacuum enclosure is pumped down to a low pressure typically of the order of 10xe2x88x925 Torr. The maintenance of an appropriate vacuum pressure within the vacuum enclosure is important for the integrity of the implant process. An important mechanism for the control of a positively charged ion beam is so called self neutralisation of the beam. In the absence of any neutralisation, the beam is essentially positively charged so that the ions of the beam tend to electrostatically repel each other, causing the beam to defocus, and in extreme cases to collapse. Self neutralisation arises because energetic beam ions cause electrons to become dissociated from low energy residual gas atoms within the vacuum enclosure, and these low energy electrons then have the effect of at least partially neutralising the beam potential so that the tendency of the beam to blow up through the effects of space charge is reduced.
If the vacuum pressure within the enclosure through which the ion beam is passing is too low, insufficient low energy electrons are produced from residual gas atoms, so that the beam is inadequately neutralised.
On the other hand, if the residual pressure within the vacuum enclosure is too high, collisions with beam ions may produce energetic residual gas atoms and ions which can result in undesirable contamination of the process wafer. Accordingly, it is important to ensure that the vacuum pressure is maintained at an appropriate level during implanting, that is to say it is important to monitor the performance of the vacuum system of the implanter. Appropriate vacuum levels are maintained within the implanter vacuum enclosure by means of a combination of turbo pumps and cryo pumps of kinds well known to workers experienced in vacuum systems.
A complicating factor in ion implantation is the tendency of the ion implantation process itself to produce bursts of outgassing from the wafers being implanted, with consequential rises in residual gas pressure, at least within the process chamber containing the wafers being implanted. This outgassing is caused by the sputtering or xe2x80x9cburning offxe2x80x9d of layers of photoresist applied to wafers prior to implanting in order to define those regions of the wafer surface which are to receive a dose of implanted ions during the implant cycle. This outgassing phenomenon is well know to those experienced in the field of ion implanters for manufacturing semiconductor devices.
Clearly, the vacuum system of the implanter must be capable of maintaining an appropriate mean vacuum pressure within the enclosure of the ion implanter, and in particular the process chamber, in spite of the bursts of outgassing during the implant process.
Nevertheless, outgassing typically produces sudden rises in residual gas pressure, particularly in the process chamber followed by a fall in residual pressure when outgassing ceases and the vacuum pumps bring the residual pressure back down again. The resulting peaks in residual pressure make it difficult to monitor the overall performance of a vacuum system and, hitherto, the first indication of poor performance of a vacuum system, that is poor dynamic pumping, is when wafers which have been processed by the implanter begin to fall below the necessary quality criteria.
An object of the invention is to provide a method of obtaining a performance parameter for an ion implanter, particularly one which can be used to monitor performance of a vacuum system.
Accordingly, the invention provides a method of obtaining a performance parameter for an ion implanter of the kind which operates by effecting repeated relative scanning of an ion beam over at least one wafer to be implanted to complete an implant cycle, comprising:
a) monitoring vacuum pressure in a vacuum chamber of the implanter to identify pulses of said pressure caused by outgassing from the wafer surface during respective scans or groups of scans of said repeated scanning, and
b) integrating pressure values during said identified pulses to provide pulse pressure integral values for respective said pulses, said integral values providing said performance parameter.
It has been found that by identifying and integrating pressure pulses detected in a vacuum chamber of the implanter, a useful parameter is provided for monitoring the performance of the implant process and of the implanter itself.
The pulse integral values may be monitored to determine whether the values exhibit a selected trend indicating a reduction in performance of the vacuum system of the implanter. This trend may be exhibited when the pulse integral values exceed a running mean of said values by more than a predetermined amount. Alternatively, the trend may be exhibited when the pulse integral values are increasing relative to a base line. The base line used may not be constant throughout an implant cycle, but may be varied during said scanning to compensate for variations in the amount of outgassing per scan or group of scans of the ion beam over a wafer during an implant cycle. In particular, the rate of outgassing from the wafer typically declines during an implant cycle as the material in the photoresist which is expelled during implanting becomes exhausted in the photoresist layer. The resulting decline in the pulse integral values during an implant cycle could mask a reduction in the performance of the vacuum system for example.
Preferably, said pulses of said pressure are identified by comparing the monitored vacuum pressure with a threshold pressure value, and the pressure values are integrated for the duration of the pulse when said monitored pressure exceeds said threshold value.
The invention also provides an ion implanter comprising a vacuum enclosure, an ion beam generator for generating in the vacuum enclosure a beam of ions for implantation, a holder for holding at least one wafer to be implanted, a scanner to effect repeated relative scanning of the ion beam over said at least one wafer to complete an implant cycle, whereby each respective scan or each of a group of said scans of said beam over said at least one wafer can cause a corresponding burst of outgassing from the wafer surface, a vacuum pressure detector providing electrical signals indicative of vacuum pressure in the vacuum enclosure, a vacuum pressure monitor responsive to said electrical signals to identify pulses of said pressure caused by said bursts of outgassing, an integrator to integrate each of said pressure pulses to provide respective pulse pressure integral values, and an implant performance monitoring system receiving said pulse pressure integral values as a performance parameter.